The present invention relates to refractometers, more particularly temperature compensated, hand-held refractometers, and to methods of aligning optically critical components thereof.
Temperature compensated, hand-held refractometers are well known in industry for measuring the refractive index of a test substance. Since the refractive index of a liquid substance is related to the concentration of elements within the substance, the units of a refractometer scale may be calibrated to indicate quantities which are of practical importance. For example, the sugar concentration in a soft drink or the lubricant concentration in machine tool cutting fluid may be readily measured using a suitably calibrated refractometer.
A common arrangement of optical elements in a temperature compensated hand-held refractometer is described in U.S. Pat. No. 3,329,060. The optical elements include a critical angle prism, a compensating prism or wedge, an objective lens, a reticle, and an eyepiece; a temperature responsive bimetallic member moves the compensating wedge to adjust for ambient temperature variations. It is known that temperature compensation may also be achieved by using a temperature responsive member to move the objective lens, as taught in U.S. Pat. Nos. 3,279,309 and 3,625,620, or the reticle, as taught in U.S. Pat. Nos. 2,619,003 and 3,447,875. Alternatives which do not rely upon mechanically moving parts to effect temperature compensation include refractometers having a liquid filled compensating prism, as disclosed in U.S. Pat. No. 3,267,795. Regardless of the method of temperature compensation, alignment of the optical elements, excluding the eyepiece, is necessary for accurately reporting refractive values.
Prior art refractometers require that alignment be carried out after the optical elements have been placed inside the refractometer housing. In commercial models sold for over twenty years, as well as in currently available models known to applicant, the critical angle prism is affixed in the housing and the other elements are placed in the housing as an unaligned subassembly; alignment is subsequently effected using externally accessible adjustment screws. FIG. 1 shows a prior art refractometer subassembly with a metal strip 10, reticle mount 16 for holding a reticle 18, lens mount 26 for holding an objective lens 28, and compensating wedge mount 30 for holding a compensating wedge 32. Compensating wedge mount 30 is fastened to one end of a temperature responsive member 34, while the other end of member 34 is fastened to strip 10.
Various advantages of the present invention distinguish it from the prior art. Because the subassembly frame is a one piece unit, it may be formed from plastic in a single molding operation. By providing a reference surface on the frame for locating and supporting a critical angle prism, complete and even permanent alignment of the optically critical elements can be achieved before they are installed within the refractometer housing. Thus, if the housing is hermetically sealed to protect the instrument from possible immersion in a test fluid, the invention eliminates the need to access external adjustment screws which may only be reached by breaking the seal.
Accordingly, it is an object of the present invention to allow for complete and durable pre-alignment of the optical elements of a temperature compensated hand-held refractometer prior to installing them as a subassembly inside the refractometer housing.
This and other objects of the invention will be apparent to those skilled in this art from the following detailed description of the invention and the accompanying figures.